1. Field of the Invention
The present invention relates to a secondary battery cell protection circuit. More particularly, the invention relates to a circuit for protecting a secondary battery cell such as a lithium-ion battery that suspends charging of the battery when overcharge of the battery is detected.
2. Description of the Related Art
Compared to a nickel-cadmium and a nickel-hydrogen battery, a lithium-ion battery has an approximately three times higher operating voltage, and double the energy density. Volume energy density of a lithium-ion battery is higher than that of a nickel-cadmium and a nickel-hydrogen battery. Therefore, the size of a lithium-ion battery is smaller, and the weight of the battery is lighter than other batteries having the same energy. Thus, a lithium-ion battery is used for a portable electronic device such as a video camera, a cellular phone, a PHS, and a laptop computer.
For a lithium-ion battery, a protection circuit is used for safety and in order to maximize the performance of the battery. The protection circuit detects overcharge of the battery and suspends charging of the battery. FIG. 2 shows an example of conventional protection circuits for secondary batteries. In FIG. 2, a lithium-ion battery 14 is connected between a positive power-supply terminal 10 and a negative power-supply terminal 12. The positive terminal 10 is connected to an inverting input terminal of a comparator 18 located in an integrated circuit 16. A standard voltage VTH1 is provided for detecting overcharge of the battery, and is supplied to a non-inverting input terminal of the comparator 18 from a constant voltage source 20. An output signal of the comparator 18 is high-level when the voltage of the terminal 10 is lower than the standard voltage VTH1, and is low-level when the voltage of the terminal 10 is higher than VTH1.
The output signal of the comparator 18 is provided to a base of an npn-transistor Q1. The transistor Q1 has its emitter grounded and its collector connected to a collector of a pnp-transistor Q2. The transistor Q2 is diode-connected by connecting its collector and its base together. The pnp-transistor Q2 also forms a current-mirror circuit with its base connected to a base of a pnp-transistor Q3. Each emitter of the pnp-transistor Q2 and the pnp-transistor Q3 is connected to the power-supply terminal 10. A collector of the pnp-transistor Q3 is connected to an output terminal 22, and the output terminal 22 emits an overcharge detection signal. The collector of the transistor Q3 is also connected to one end of a resistor R1 and to a gate of an overcharge preventing MOS transistor Q5. The other end of the resistor R1 is connected to the negative power-supply terminal 12. The overcharge detection signal is supplied to the gate of the overcharge preventing MOS transistor Q5.
In addition, the power-supply terminal 10 is connected to an over-discharge detection unit 24 located in the integrated circuit 16. The over-discharge detection unit 24 outputs an over-discharge detection signal. When the voltage of the terminal 10 is higher than or equal to a standard voltage VTH2, the over-discharge detection signal is high-level. When the voltage of the terminal 10 is lower than the standard voltage VTH2, the signal is low-level. This over-discharge detection signal is then supplied to a gate of a discharge preventing MOS transistor Q4 outside the integrated circuit 16.
A negative pole of the lithium-ion battery 14 is connected to a source of the discharge preventing MOS transistor Q4. A drain of the MOS transistor Q4 is connected to a drain of the overcharge preventing MOS transistor Q5, and a source of the MOS transistor Q5 is connected to the negative power-supply terminal 12. Further, the potential at the negative pole of the battery 14 is set to the ground level. Since the gate is connected to a substrate at each of the MOS transistors Q4 and Q5, body-diodes D4 and D5 are formed between the drain and the source of the respective MOS transistors Q4 and Q5.
The over-discharge detection signal is high-level when the voltage of the power-supply terminal 10 is higher than or equal to the standard voltage VTH2, so that the over-discharge prevention MOS transistor Q4 is activated unless the lithium-ion battery 14 is over discharged. Further, the overcharge detection signal is high-level when the voltage of the terminal 10 is lower than or equal to the standard voltage VTH1. The overcharge preventing MOS transistor Q5 is activated unless the battery 14 is overcharged.
In addition, if the battery 14 is overcharged while charging the battery through a charger circuit that is connected between the terminals 10 and 12, the output signal of the comparator 18 becomes low-level, and the transistor Q1 is deactivated. Consequently, the transistors Q2 and Q3 are deactivated, and the output terminal 22 outputs a low-level overcharge detection signal. As a result, the overcharge preventing MOS transistor Q5 is deactivated, and the battery 14 is no longer charged.
In the above-described prior circuit, after the lithium battery 14 is overcharged and the charging process is stopped, a load such as a video camera, a cellular phone, a PHS, a laptop computer may be connected to the power-supply terminals 10 and 12. When the load is connected, the over-discharge prevention MOS transistor Q4 is active, and the body-diode D5 of the overcharge detection MOS transistor Q5 turns on itself to allow an electrical current to flow from the battery 14 through the body-diode D5 to the load.
Though it is not a problem that the discharged current from the battery 14 flows through the loads, the temperature of the overcharge prevention MOS transistor Q5 increases because of the current flowing not through the overcharge prevention MOS transistor Q5 but through the body-diode D5. Thus, the MOS transistor Q5 deteriorates due to the increase in temperature thereof.